Connecting a plurality of computers to constitute a LAN allows for sharing of information such as files and data, allows for sharing of peripheral equipment such as a printer, and allows for information exchange such as the transfer of electronic mail and/or data content.
Conventionally, a fiber-optic cable, a coaxial cable, or a twisted-pair cable has been used to provide a wired LAN. In such a case, however, line installation work is required, which makes it difficult to readily construct a network. In addition, the cabling is messy. After the LAN is constructed, the movement range of equipment is limited by the cable length, which is inconvenient.
Accordingly, a scheme for freeing users from the wiring of a wired-system LAN, wireless networks are gaining attention. Since the wireless networks can omit a majority of wire cables in work spaces such as offices, communication terminals such as personal computers (PCs) can be moved with relative ease.
In recent years, in conjunction with an increased speed and reduced cost of wireless LAN systems, demands thereon have increased considerably. In particular, recently, in order to construct a small-scale wireless network with multiple pieces of electronic equipment available around people to perform information communication, the incorporation of a personal area network (PAN) is under consideration. For example, frequency bands, such as a 2.4 GHz band, a 5 GHz band, and so on, that do not require a license of a regulatory agency, are utilized to define different wireless communication systems.
A method that is typically used to configure a local area network using a wireless technology is that one apparatus that serves as a controlling station, called an “access point” or a “coordinator”, is provided in an area to form a network under the centralized control of the controlling station.
In a wireless network in which an access point is arranged, an access control method based on bandwidth reservation is widely employed. That is, in the method, when one communication apparatus transmits information, it first makes a reservation with the access point for a bandwidth required for the information transmission and uses a transmission path so that the information transmission does not collide with information transmission of another communication apparatus. Thus, with the arrangement of the access point, the communication apparatuses in the wireless network can perform wireless communication in a synchronous manner, i.e., in synchronization with each other.
However, in a wireless communication system in which an access point exists, when asynchronous communication is performed between a transmitting-side communication apparatus and a receiving-side communication apparatus, wireless communication that always goes through an access point is required. Thus, there is a problem in that the use efficiency of the transmission path is reduced by half.
As opposed to it, as another method for forming a wireless network, “ad-hoc (ad-hoc) communication”, in which terminals perform communication directly with each other in an asynchronous manner, has been devised. In particular, in a small-scale wireless network constituted by a relatively small number of clients located in the neighbor, the ad-hoc communication that allows arbitrarily terminals to perform asynchronous wireless communication directly with each other without the use of a specific access point is considered to be appropriate.
Examples of typical standards related to the wireless network include IEEE (The Institute of Electrical and Electronics Engineers) 802.11 and IEEE 802.15.3. With regard to the IEEE 802.11 standard, various wireless communication systems, such as an IEEE 802.11a standard, IEEE 802.11b, and so on, are available, depending on a difference in a wireless communication system and a frequency band used.
In addition, recently, as a wireless communication system for achieving short-distance ultrahigh-speed transmission, a system, called “ultra-wide band (UWB) communication”, for performing wireless communication by sending information over a series of considerably weak impulses has been gaining attention, and it is anticipated that the system is put to practical use (e.g., refer to Non Patent Document 1).
The UWB transmission system can be divided into two types, namely, a DS-UWB system, in which the spreading speed of a DS information signal is increased to the limit, and an impulse-UWB system, in which an information signal is formed of a series of impulse signals having a very short period, i.e., a period of about several hundred picoseconds, and the series of signals is transmitted/received. Either of the systems performs transmission/reception by spreading the signals at an ultra-high frequency spectrum as high as 3 GHz to 10 GHz, thereby achieving high-speed data transmission. The occupied bandwidth is on the order of GHz such that a value obtained by dividing the occupied bandwidth by the center frequency (e.g., 1 GHz to 10 GHz) thereof is substantially “1” and is an ultra-wide band compared to a bandwidth typically used in a wireless LAN using the so-called a W-CDMA or cdma2000 system or an SS (spread spectrum) or OFDM (orthogonal frequency division multiplexing) system.
For example, a method for performing communication by forming a piconet between wireless communication apparatuses that perform ultra-wide band wireless communication is being standardized in IEEE 802.15.3 standardization work.
Now, details of conventional wireless networking will be described in connection with an example of IEEE 802.11.
IEEE 802.11 networking is based on the concept of a BSS (Basic Service Set). The BSS is constituted by two types of BSS, namely, a BSS defined by an infrastructure mode in which a master, such as an AP (access point: controlling station), exists and an IBSS (Independent BSS) defined by an ad-hoc mode in which only a plurality of MTs (mobile terminals: mobile stations) constitute a network.
Infrastructure Mode
The operation of IEEE 802.11 in infrastructure mode will be described with reference to FIG. 24. For the BSS in infrastructure mode, an AP is essential in the wireless communication system to perform coordination.
The AP organizes, as a BSS, a range in which radio waves reach in the neighbor of the ‘self’ station, and provides a “cell” referred to in the so-called cellular system. MTs that exist in the neighbor of the AP are covered by the AP to thereby join the network as members of the BSS. Thus, the AP transmits a control signal, called a beacon, at appropriate time intervals. In turn, an MT that can receive the beacon recognizes that the AP exists in the neighbor, and further establishes a connection with the AP.
In the example shown in FIG. 24, a communication station STA0 operates as an AP and other stations STA1 and STA2 operate as MTs. The communication station STA0, which serves as an AP, transmits a beacon (beacon) at regular time intervals, as depicted in the chart at the right-hand side in the figure. The transmission time of a next beacon is indicated in the beacon in the format of a parameter called target beacon transmit time (TBTT: target beacon transmit time). When time reaches the TBTT, the AP operates a beacon transmission procedure.
Upon receiving the beacon, each neighboring MT decodes a TBTT field therein to thereby allow the recognition of next beacon transmission time. Thus, in some cases (when there is no need for reception), the neighboring MT may power off the receiver to enter a sleep state until receiving a next TBTT or a TBTT subsequent thereto.
Ad-Hoc Mode
The operation based on IEEE 802.11 during the other mode, i.e., the ad-hoc mode, will now be described with reference to FIGS. 25 and 26.
With respect to an IBSS in ad-hoc mode, after negotiating with each other, a plurality of MTs autonomously define an IBSS. After the IBSS is defined, the MT group specifies the TBTT at regular time intervals. Each MT refers to the clock of the self station to recognize the arrival of the TBTT. Then, when recognizing that nobody has transmitted a beacon yet after a delay of random time, the MT transmits a beacon.
FIG. 25 shows a state in which two MTs constitute the IBSS. In this case, any one of the MTs belonging to the IBSS transmits a beacon every time the TBTT arrives. Beacons transmitted from the MTs may collide with each other.
Also, in the IBSS, the MTs may enter the sleep state in which the transceiver is powered off, as needed. FIG. 26 shows a signal transmission/reception procedure in such a case.
In the IBSS in IEEE 802.11, when the sleep mode is used, some period of time after the TBTT is defined as an ATIM (announcement traffic indication message) window. During the period of the ATIM window, all MTs belonging to the IBSS perform reception processing. During this period of time, in essence, an MT that is operating in the sleep mode can also perform reception.
When the self station has information addressed to someone, each ME transmits an ATIM packet to the one after transmitting a beacon in the time period of the ATIM window, so that the reception side is notified that the self station has information addressed to the one. An MT that has received the ATIM packet keeps the receiver in operation until reception from the station that has transmitted the ATIM packet is completed.
In the example shown in FIG. 26, three MTs STA1, STA2, and STA3 exist in the IBSS. In the figure, upon the arrival of the TBTT, each of the MTs STA1, STA2, and STA3 operates a backoff timer, while monitoring the medium state for a random time. In the illustrated example, the timer of the MT STA1 expires earliest and the MT STA1 transmits a beacon. Since the MT STA1 transmits a beacon, the MTs STA2 and STA3 that receive the beacon are adapted not to transmit beacons.
In the examples shown in FIG. 26, the MT STA1 holds transmission information addressed to the MT STA2 and the MT STA2 holds transmission information for the MT STA3. In this case, after transmitting or receiving the beacons, the MTs STA1 and STA2 operate the backoff timers again while monitoring the respective medium states for a random period of time. In the illustrated example, since the timer of the MT STA2 expires earlier, the MT STA2 first transmits an ATIM message to the MT STA3. Upon receiving the ATIM message, the MT STA3 feeds back information indicating the reception to the MT STA2 by transmitting an ACK (acknowledge) packet. When the MT STA 3 finishes the transmission of the ACK, the MT STA1 further operates the backoff timer while monitoring the state of each medium for a random period of time. When the timer expires, the MT STA1 transmits an ATIM packet to the MT STA2. The MT STA2 performs feedback by returning an ACK packet indicating the reception to the MT STA1.
In a period after the ATIM packet and the ACK packet are exchanged in the ATIM window, similarly, the MT STA 3 operates the receiver to receive information from the MT STA2 and the station STA2 operates the receiver to receive information from the MT STA1.
In the above-described procedure, a communication station that does not receive an ATIM packet in the ATIM window or that does not hold transmission information addressed to someone can power off the transceiver until the next TBTT to reduce power consumption.
The present inventors consider that such a wireless networking operation has mainly three problems.
The first problem is collision due to a change in the radio-wave propagation environment.
For example, suppose a situation in which systems that have formed respective networks approach each other, as shown in FIG. 27. In the upper section in FIG. 27, a network constituted by communication stations STA0 and STA1 and a network constituted by communication stations STA2 and STA3 exist in ranges where radio waves do not reach due to a shield, such as a wall and/or door, which are not shown. Thus, the communication stations STA0 and STA1 communicate with each other, and, completely independently thereof, the communication stations STA2 and STA3 communicate with each other. The beacon transmission timing of each communication in this case is shown at the right side in the upper section in FIG. 27.
In such a communication environment, suppose a case in which the door that has been providing a shield between the networks opens and those stations, which have not recognized each other, recognize each other. The lower section of FIG. 27 shows a case in which the stations STA0 and STA1, which have been communicating completely independently, and the stations STA2 and STA3 are put in state in which they can communicate with each other. In this case, as shown at the right side in the lower section in FIG. 27, beacons from the respective stations collide with each other.
With the widespread use of information equipment such as personal computers (PCs), it can be presumed that communications stations are ubiquitous in a work environment where a large number of pieces of equipment coexist in an office. In such a situation, network construction based on IEEE 802.11 will now be discussed.
When a network is construed in the infrastructure mode, the issue is selection as to which communication station should be operated as the AP (i.e., coordinator). According to IEEE 802.11, an MT included in a BSS communicates with only a communication station belonging to the same BSS and the AP works as a gateway for another BSS. In order to conveniently network the entire system, it is necessary to make a schedule for the system of the entire network in advance. However, in an environment such as a home network where the user gets across between communication stations or the radio-wave propagation environment frequently changes, it is impossible to overcome issues, such as the issue of selecting, as the AP, a communication station located at which position and the issue of how to re-constitute the network when the AP is powered off. Thus, it can be assumed that it is preferable to be able to construct a network without any coordinator, but such a demand cannot be met in the IEEE 802.11 infrastructure mode.
The second problem is collision resulting from a network environment change due to a mobile terminal.
As in the case of FIG. 27, a situation in which systems that have formed respective networks approach each other will be discussed with reference to FIG. 28. The operation states of communication stations STA0 to STA3 are analogous to the case shown in FIG. 27. In such a communication environment, suppose a case in which a user moves his/her communication station to thereby cause communication stations, which have not recognized each other, to recognize each other, because of the presence of a communication station STA4.
As shown in the lower section in FIG. 28, when each communication is put in a state that allows transmission and reception, beacons from the respective stations except the station STA4 collide with each other. In connection with this problem, in the IEEE 802.11 standard, the station STA4 can receive both signals from a first network (IBSS-A) and signals from a second network (IBSS-B). When the information of beacons is mutually read, the networks break down. As a result, the station STA4 needs to operate in accordance with rules for the IBSS-A and the IBSS-B, and the possibilities of beacon collision and ATIM packet collision exist in anyway. Although it is desired that a network can be constructed without a need for a coordinator (as described above), such a demand cannot be met in the IEEE 802.11 infrastructure mode.
The third problem is a configuration for managing a network in which the loads of communications are low.
Now, neighboring-station information (a neighbor list) of a communication station having a coordinator function will be described. Typically, a communication station having a coordinator function transmits a beacon to notify neighboring stations about network information. Since the coordinator performs overall management of the network, the loads of the neighboring stations are reduced. In contrast, when the first and second problems in the networking are considered, it is desirable that a network without a coordinator be constructed in, particularly, a home network. In such a case, each communication station needs to have a neighbor list. However, as the number of neighboring stations that each neighboring station can handle increases, the load of each neighboring station increases. This can lead to a load on the entire network. Accordingly, it is necessary to consider a low-load configuration for managing a network.
[Non Patent Document 1]
Nikkei Electronics, issue of Mar. 11, 2002, “First Cry of Wireless Revolution, Ultra Wideband (Ubugoe-wo-ageru Musen-no-Kakumeiji Ultra Wideband)” (pp. 55-66).